This proposal addresses the need for predicting multicomponent vapor concentrations of evaporating organic solvent mixtures from the measurement of a single component of the vapor phase and quantitative analysis of the bulk liquid. Existing solutions to this problem suffer from one or more shortcomings due to: (1) inappropriate or unrealistic simplifying assumptions, (2) limiting restrictions not applicable to most field situations, or (3) lack of sufficient and accurate thermodynamic data for the implementation of more exact solutions. An engineering model of the processes of evaporation and dilution to breathing zone concentrations is proposed to be used with a thermodynamic model to predict the equilibrium vapor concentrations. The proposed models employ the approach of calculating relative vapor ratios to eliminate the approximation of otherwise indeterminant mass transfer parameters and a recent breakthrough in the computer prediction of the thermodynamic parameter activity coefficient, which adjusts for non-ideal liquid behavior. This model will be tested and refined in the laboratory under various air flow conditions. During Year 2 of this study it is proposed to do field testing to complete the verification of the new prediction methodology. Various schemes will also be explored to allow general field usage of the methodology without the need for a computer.